The succession and structure east of the Glaslyn River, North Wales

Abstract

The succession and structure of some mid-Ordovician sedimentary and volcanic rocks are described. Marine slates and grits are succeeded by welded and bedded rhyolitic pyroclastic rocks which are overlain by basic tuffs, lavas, and intercalated slates. A local intravolcanic unconformity cuts out over 5000 ft of strata below part of the Snowdon Volcanic Series. The fragmented debris of the missing strata forms a mixed deposit of breccias which overlies the unconformity in places. Normal faulting of Lower Caradoc age is postulated, and it is suggested that the unconformity is part of a buried volcanic collapse-structure. Timerelations suggest that the collapse was initiated by concealed intrusive activity near Aberglaslyn. The intrusion produced local thermal metamorphism and tourmalinization of the early Caradoc sediments in post-Harnagian and pre-Longvillian times.